Broken Access Control in Grape with Bearer Tokens

Broken Access Control occurs when API endpoints do not properly enforce authorization checks, allowing one user to access or modify resources belonging to another user. When Grape is used with Bearer Tokens for authentication, the vulnerability arises if token validation is performed but authorization is not consistently applied across every protected route. A common pattern is to authenticate the request using a token and set current_user, yet fail to verify that the authenticated user is permitted to operate on the targeted resource.

Consider a Grape API that authenticates via Bearer Tokens and exposes nested resource routes such as /projects/:project_id/tasks/:id. If the API only checks that a valid Bearer Token exists and maps the token to a user, but does not ensure the user belongs to the project associated with project_id, an attacker can manipulate the :id parameter to access tasks they should not see or modify. This is a BOLA (Broken Object Level Authorization) scenario, often cataloged under OWASP API Top 10 as Broken Object Level Authorization and a contributor to incidents like CVE-2021-28071-like access bypass patterns.

In Grape, this can occur when route-level authorization is omitted or inconsistently applied. For example, using a before block to authenticate but skipping per-resource ownership checks means any authenticated user can iterate over IDs and probe for accessible objects. Attackers may use unauthenticated scanning to discover endpoints, then test token validity and enumerate accessible resources. The API might return 200 with data or 404 to hide existence, but without consistent authorization, the difference leaks information and enables privilege escalation or data exposure.

Another vector involves role-based access where token claims include a role, but Grape endpoints do not validate the role before performing sensitive actions. If an administrative action such as DELETE /projects/:id relies solely on the presence of a token and does not verify that the token’s associated user has the admin role, a compromised or misconfigured token can lead to critical security impacts. This ties into BFLA (Business Logic Flaw Abuse) and Privilege Escalation, where the business logic incorrectly trusts the caller once authentication succeeds.

Because Grape allows flexible route and helper organization, developers must ensure that every resource-level operation includes explicit authorization checks that compare the authenticated user’s identity or permissions against the resource’s ownership or access control lists. Relying on authentication alone, even when using Bearer Tokens, is insufficient to prevent Broken Access Control.

To remediate Broken Access Control when using Bearer Tokens in Grape, combine robust authentication with explicit, per-request authorization checks. Always validate the token, set the current user, and then enforce that the user is authorized for the specific resource and action.

Below is a secure Grape API example that demonstrates these principles. It includes token validation, user scoping, and resource ownership checks before allowing access.

require 'grape'
require 'json_web_token' # hypothetical JWT helper

class MyAPI < Grape::API
  format :json

  helpers do
    def authenticate!
      token = request.headers['Authorization']&.to_s&.sub(/^Bearer\s/, '')
      error!('Unauthorized', 401) unless token
      @current_user = User.find_by_auth_token(token) # validates token and fetches user
      error!('Unauthorized', 401) unless @current_user
    end

    def authorize_project_access!(project_id)
      project = Project.find(project_id)
      # Ensure the authenticated user has permission for this specific project
      unless project.user_id == current_user.id || current_user.admin?
        error!('Forbidden', 403)
      end
      project
    end

    def current_user
      @current_user
    end
  end

  before do
    authenticate!
  end

  resources :projects do
    get ':id' do
      project = authorize_project_access!(params[:id])
      { id: project.id, name: project.name }
    end

    resources :tasks do
      get ':id' do
        task = Task.find(params[:id])
        authorize_project_access!(task.project_id)
        { id: task.id, title: task.title }
      end

      post do
        project = authorize_project_access!(params[:project_id])
        # further ownership or policy checks can be applied here
        { message: 'Task created' }
      end
    end
  end
end

Key points in this remediation:

• authenticate! extracts the Bearer Token from the Authorization header and sets @current_user only if the token is valid.
• authorize_project_access! explicitly checks that the current user either owns the project or has admin rights before allowing access.
• Each resource action calls authorize_project_access! with the relevant resource ID, ensuring per-request authorization tied to the specific object.
• Admin checks are explicit and role-based, preventing privilege escalation via token claims alone.

For endpoints that operate on non-resource identifiers (e.g., UUIDs not directly tied to a project), apply similar scoping: validate that the requested entity is within the set the user is allowed to access, using policy objects or service classes as needed. Avoid relying on token claims for authorization decisions unless those claims are verified and constrained server-side.